Prosthetic heart valves constructed of glycerol-treated human dura mater have been used as replacement cardiac valves. Morphologic studies were made of changes developing in these valves after being in place for periods for up to 4 years. Dura mater valves appears to have less frequent platelet aggregates than did porcine valves. Other changes in dura mater valves, including calcific deposits, and degeneration of collagen appeared similar to those in porcine valves. These alterations represent consequences of the gradual enzymatic and mechanical breakdown of valvular connective tissue under conditions in which synthesis of new components is not possible. Thus, these changes impose limitations under long-term durability of the dura mater valves.